Polyethylenenaphthalate (PEN) has got very good mechanical strength and heat resistance. Its known manufacturing processes, however, have not been competitive.
Polyethylenenaphthalate is made from 2,6-naphthalenedicarboxylic acid, which is made by oxidizing 2,6-dimethylnaphthalene. Nowadays 2,6-dimethylnaphthalene is separated from hydrocarbon mixtures. In this way only small amounts are obtained. Furthermore also the separation and purification is difficult.
It has also been suggested to manufacture 2,6-dimethylnaphthalene by cyclization of 1-(p-tolyl)-2-methylbutane and/or 1-(p-tolyl)-2-methylbutene.
The manufacturing of 2,6-dimethylnaphthalene has been described in many patents. In some of these patents also the manufacturing of 1-(p-tolyl)-2-methylbutane and/or 1-(p-tolyl)-2-methylbutene is described. In some patents also the use of chromium oxide/aluminum oxide and its impregnation with potassium is mentioned.
In GB-1448136 (1976) the manufacturing of 1-(p-tolyl)-2-methylbutane and/or 1-(p-tolyl)-2-methylbutene in many different ways is described. In most cases one component is p-xylene, the methyl group of which is alkylated with butene using a basic catalyst. In three examples the manufacturing of 2,6-dimethylnaphthalene using a Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3 -catalyst poisoned with potassium is described. In the claims the amount of impregnated alkali metal oxide is 0.1-20 wt. % but in the examples the amounts of potassium are 2.1 and 3.5 wt. %. However, 3.1% and 5.0% of the total amount of dimethylnaphthalene is 2,7-dimethylnaphthalene which is very difficult to separate.
In EP430714A (1990) a chromium oxide/aluminum oxide catalyst poisoned with potassium is mentioned in one example but in the claims only the oxides are mentioned but not the amounts. In one example the components K:Cr:AL are in the proportions 7:11:100. In the same example the proportion of 2,6-dimethylnaphthalene of the total amount of dimethylnaphthalenes is 98.6%.
In EP-362507A (1989) p-xylene, 1-butene and carbon monoxide are used as raw-materials when making 1-(p-tolyl)-2-methylbutane and/or 1-(p-tolyl)-2-methylbutene. When making the 2,6-dimethylnaphthalene Cr.sub.2 O.sub.3 -5% K.sub.2 O-Al.sub.2 O.sub.3 is used as catalyst The selectivity of 2,6-dimethylnaphthalene was reported to be 71%. Nothing is mentioned about the purity of the product. In the claims only aluminum oxide and silicon oxide are mentioned.
EP-362651A (1989) is almost the same as EP-362507A regarding the dehydrocyclization reaction. The only difference is that m-xylene, propylene and carbon monoxide are used for making 2,4-dimethyl-isobutyrophenone, which is cyclisized to 2,6-dimethylnaphthalene.
In the patents mentioned above nothing is said about C12-alkylindanes and -indenes which are very difficult to separate because they are boiling at almost the same temperature as the dimethylnaphthalenes.